FIG. 1 is a block diagram showing a configuration of a conventional basic PID control system. In FIG. 1, the reference numeral 224 designates a wafer placed in a constant temperature oven not shown; 225 designates a thermocouple for detecting the temperature near the wafer 224; 226 designates a PID computation section for receiving the temperature detected by the thermocouple 225 along with a steady-state reference temperature, and for outputting a manipulated variable that will converge the detected temperature to the steady-state reference temperature; 227 designates a control section for carrying out control in response to the manipulated variable; 228 designates a heater placed near the wafer 224; 229 designates a power supply; and 230 designates a control loop for connecting the heater 228 and power supply 229 to the control section 227.
Next, the operation will be described.
Once the steady-state reference temperature is set, the PID computation section 226 outputs a manipulated variable based on the PID control in response to the temperature difference of the temperature detected by the thermocouple 225 with respect to the steady-state reference temperature, and controls the energized duration of the heater 228 in response to the manipulated variable of the control section 227.
Accordingly, the conventional basic PID control system can control the temperature at and around the position of the thermocouple 225 such that it is stabilized at the steady-state reference temperature.
However, as the size of the wafer 224 to be controlled increases, there arises a problem of being unable to control the temperature of the entire wafer 224 uniformly.
In view of this, Japanese patent application laid-open No. 7-96168 discloses a technique that divides the space associated with the temperature control into zones, and carries out the PID control of the individual zones separately. The PID control, which is performed separately for the individual targets to be controlled, causes difference in timing for the individual zones to reach the steady-state reference temperature depending on their environment and positional relationships, even if the same steady-state reference temperature is used for the control. Thus, it also discloses the technique for controlling the start and end timing of the PID control of the individual zones.
FIG. 2 is a block diagram showing a configuration of a second conventional PID control system disclosed in the foregoing Japanese patent application laid-open No. 7-96168. In FIG. 2, the reference numeral 231 designates a process controller for outputting a preset temperature in accordance with a specified program; 232 designates a ramp signal generator for outputting a ramp waveform having the preset temperature as its final temperature; 233 designates a PID controller for receiving the ramp waveform as the steady-state reference temperature, and for calculating and outputting a manipulated variable; and 234 designates a furnace, the temperature of which is controlled by the PID controller 233. Reference numerals 235 designate a plurality of intra-furnace temperature sensors. The reference numeral 236 designates an initializing memory for presetting a comparison reference temperature; 237 designates a time difference measuring circuit for measuring timings at which the temperatures detected by the individual intra-furnace temperature sensors 235 agree with the comparison reference temperature, and for outputting the time differences; and 238 designates a time difference table memory for storing data for controlling the generating timing of the ramp signal of the ramp signal generator 232 in response to the time difference.
Next, the operation will be described.
The process controller 231 outputs the preset temperature in a state in which the comparison reference temperature is preset in the memory 236. In response to the output, the ramp signal generator 232 generates the ramp waveform having the preset temperature as the final temperature, and the PID controller 233, receiving the ramp waveform as the steady-state reference temperature, calculates and outputs the manipulated variable. As a result, the temperature of the furnace 234 varies toward the preset temperature. If the temperatures detected by the individual intra-furnace temperature sensors 235 agree with the comparison reference temperature in the course of the temperature variation, the time difference measuring circuit 237 measures the coincident timings, and supplies the time differences between them to the time difference table memory 238. The time difference table memory 238 selects the table data that will compensate for the time differences, and supplies them to the ramp signal generator 232.
Subsequently, in response to the preset temperature the process controller 231 outputs, the ramp signal generator 232 adjusts the output start timings of the time ramp waveforms using the time preset by the difference table memory 238. Thus, in response to the ramp waveforms, the temperature of the furnace 234 is controlled to the preset temperature. Consequently, it can control the temperature of the single furnace 234 by carrying out a plurality of PID controls, and match the timings theoretically at which the temperatures reach the steady-state reference temperature.
FIG. 3 is a block diagram showing a configuration of a control system using the conventional control unit. In FIG. 3, the reference numeral 261 designates a control unit having a PID control function; 262 designates a constant temperature oven; 263 designates a wafer placed in the constant temperature oven; 264 designates a heater for controlling the internal temperature of the constant temperature oven 262 in response to the manipulated variable fed from the control unit 261; and 265 designates a temperature sensor for detecting the temperature near the wafer 263.
Next, the operation will be described.
The control unit 261 having the PID control function receives the temperature measurement value the temperature sensor 265 detects, carries out the calculation based on the PID control function such that the measurement value matches the reference temperature preset value, and calculates the manipulated variable. The manipulated variable is supplied to the heater 264. The heater 264 controls the internal temperature of the constant temperature oven 262 by varying the energized duration in accordance with the manipulated variable. Thus, the control is carried out in such a manner that the temperature of the wafer 263 agrees with the preset reference temperature.
Incidentally, as the size of the wafer 263 to be controlled increases, it becomes more difficult to control the temperature of the entire wafer 263 uniformly. In this case, it will be possible to divide the wafer 263 to be controlled into a plurality of regions, and utilize the foregoing control unit 261 having the PID control function for each of these regions. Assigning the temperature sensor 265 and heater 264 to each of the regions to carry out the PID control individually enables the temperatures of the individual regions to match the preset reference temperature.
However, such a control method has a problem of being unable to carry out suitable control because of interference between temperatures of the individual regions. Specifically, although the temperature of a particular region is controlled by the PID control via the heater of that region, the heaters of other regions have influences on the temperature of that region, thereby deteriorating the appropriate control.
To solve the problem, the control system as shown in FIG. 4 is used, for example. In FIG. 4, the reference numeral 271 designates a control unit with the PID control function; 272 designates a constant temperature oven; 273 designates a wafer placed in the constant temperature oven 272; 274-1 designates a heater for controlling the temperature of a first region 273-1 in the constant temperature oven 272 in response to a ch1-manipulated variable; 274-2 designates a heater for controlling the temperature of a second region 273-2 in the constant temperature oven 272 in response to a ch2-manipulated variable; 275 designates a temperature sensor for detecting the temperature near the wafer 273; 281 designates a reference value preset section for presetting the reference temperature of the wafer 273; 282 designates an adder for calculating a deviation from the temperature measurement value the temperature sensor 275 detects and the preset value the reference value preset section 281 presets; 283 designates a PID computation section for performing the PID computation on the deviation the adder 282 calculates, and for outputting the manipulated variable; 284 designates a manipulated variable limiter for putting an upper and lower limits to the manipulated variable; reference numerals 285-1 and 285-2 designate a first ratio computation section and a second ratio computation section for performing a ratio computation on the manipulated variable; 286-1 and 286-2 designate a first offset computation section and a second offset computation section for performing an offset computation on the manipulated variable; the reference numeral 287 designates a branch point for branching the manipulated variable output from the manipulated variable limiter 284; and 288 designates a parameter preset section such as a keyboard for manually presetting parameters of the manipulated variable limiter 284 and the like.
Next, the operation will be described.
The control unit 271 receives the temperature measurement value the temperature sensor 275 detects, and carries out the following processing therein. First, the adder 282 calculates the deviation from the preset value of the reference temperature preset by the reference value preset section 281, and the PID computation section 283 determines the manipulated variable by the PID computation. Then, the manipulated variable limiter 284 puts the upper and lower limits on the manipulated variable, followed by branching it by the branch point 287.
A first branched manipulated variable becomes a ch1-manipulated variable through the first ratio computation section 285-1 and first offset computation section 286-1. Likewise, a second branched manipulated variable becomes a ch2-manipulated variable through the second ratio computation section 285-2 and second offset computation section 286-2.
The ch1-manipulated variable is supplied to the heater 274-1. The heater 274-1 controls the temperature of the first region 273-1 in the constant temperature oven 272 by varying energized duration in response to the ch1-manipulated variable, for example. Likewise, the ch2-manipulated variable is supplied to the heater 274-2, which controls the temperature of the second region 273-2 in the constant temperature oven 272 in response to the ch2-manipulated variable.
In this case, the parameters of the manipulated variable limiter 284, first ratio computation section 285-1, second ratio computation section 285-2, first offset computation section 286-1, and second offset computation section 286-2 have been set in the parameter preset section 288 in advance by a key operation at an adjustment such that the temperature of the wafer 273 is matched to the reference temperature.
In this way, by controlling the heater 274-1 placed in the first region 273-1 and the heater 274-2 placed in the second region 273-2, the temperatures of the individual regions are controlled such that they match to the reference temperature. Thus, the temperature of the entire wafer 273 is controlled such that it matches the preset reference temperature.
With the foregoing configuration, it is surely possible for the second conventional PID control system as shown in FIG. 2 to match the timings to reach the steady-state reference temperature, theoretically. However, since it varies only the start timings (end timings) of the individual PID controls, it has the following problems. First, it is difficult for it to match the timings without an additional ramp generator that continuously varies the steady-state reference temperature itself up to the final steady-state reference temperature in FIG. 2. In addition, it has a problem of introducing an overshoot or undershoot after reaching the steady-state reference temperature even if the coefficients in the individual PID controls are set to prevent the overshoot and undershoot. The overshoot or undershoot may hinder stabilizing the detected temperature at the steady-state temperature.
In other words, the second conventional PID control system holds as its data the time, differences in the timings to reach the given comparison reference temperatures in the temperature rise duration when the same steady-state reference temperature is preset for all the PID controls, and varies the timings to compensate for the time differences. It assumes in this case that the individual PID controls vary along the same curve of the temperature rise up to the steady-state reference temperature (thus, the ramp signal generator is used). However, because of the variations in the thermal conversion efficiency of the heaters in the individual PID controls and the variations in the heat dispersion characteristics of the individual regions depending on their environments, the curve of the temperature rise of the individual PID controls usually differ from one anther (that is, the time constants of the individual PID controls usually differ). Therefore if the final steady-state reference temperature is directly input to the individual PID controls, the manners of the temperature variations during the temperature rise will differ from region to region, even if the control is carried out such that the timings arriving at the comparison reference temperatures match. This will cause the temperature exchange between the regions because of the temperature gradient, which will differentiate the timings for the individual regions to reach the desired preset temperatures. As a result, the overshoot or undershoot will be introduced after reaching the preset temperature.
Although the foregoing description is made by way of example of the PID control system, other systems such as an IMC control system have similar problems.
Since the conventional control unit is configured as shown in FIGS. 3 and 4, it is likely to be able to control the temperature uniformly over the entire controlled system even if its size is large.
However, in the case where the controlled system is replaced to another controlled system, or the size of the controlled system varies, or the heaters have secular changes, or the temperature near the controlled system varies, it has a problem of being unable to control the temperature uniformly over the entire controlled system.
For example, assume in the control system of FIG. 4 that the controlled system, the wafer 273, is replaced by another type of wafer. In this case, the temperature of the first region 273-1 will be controlled appropriately because the temperature sensor 275 is placed at the first region 273-1. However, the temperature of second region 273-2 cannot be controlled suitably because of the difference in the thermal conductivity of the wafer 273 and the like. In addition, even if the temperature sensor 275 is placed at the center of the constant temperature oven 272, this will bring about inappropriate control of the temperature not only of the second region 273-2, but also of the first region 273-1.
FIG. 5 is a characteristic diagram of an internal temperature of the constant temperature oven 272 in the conventional control system as shown in FIG. 4. FIG. 5(1) is a characteristic diagram of the temperature of a first wafer, and FIG. 5(2) is a characteristic diagram of the temperature of a second wafer. Reference numerals 291 and 294 designate a reference temperature preset value that is preset by the reference value preset section 281; 292 and 295 each designate a temperature measurement value of the first region 273-1 the temperature sensor 275 detects; and 293 and 296 each designate a temperature measurement value of the second region 273-2 the temperature sensor detects.
According to the temperature characteristic of FIG. 5(1), the parameters of the manipulated variable limiter 284, first ratio computation section 285-1, second ratio computation section 285-2, first offset computation section 286-1 and second offset computation section 286-2 are set such that the temperature measurement values 292 and 293 match the reference preset temperature at the adjustment in advance. Accordingly, the temperature measurement values 292 and 293 match the reference preset temperature, resulting in appropriate control.
According to the temperature characteristic of FIG. 5(2), the control is performed on the second wafer in accordance with the parameters adjusted for the first wafer in advance. Consequently, the temperature measurement value 296 of the second region 273-2 has a certain deviation, preventing a uniform temperature over the entire wafer.
In this case, it is necessary to revise the parameters of the second ratio computation section 285-2 and second offset computation section 286-2 such that the uniform temperature control is carried out all over the second wafer. This means that the revision of the parameters must be done every time the controlled system is changed, which is very tedious. Likewise, it presents a problem of resetting the parameters to appropriate values when the environment conditions of the controlled system vary.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a control system capable of controlling the temperature of a wafer in the process of fabricating semiconductor devices such as CCD sensors by matching the arriving timings at the steady-state reference temperature by the basic control without relying on a ramp signal generator or the like in spite of employing the method of dividing the controlled system into a plurality of regions and of carrying out control of the individual regions, and by effectively preventing the overshoot and undershoot after reaching the steady-state reference temperature.
Another object of the present invention is to provide a control unit capable of carrying out the control that will match the measurement values to the preset reference value all over the controlled system without revising the parameters even if the controlled system or environmental conditions are changed.